1. Field of the Invention
The present invention relates to an optical disc comprising a light-transmitting layer and a data-recording layer, in which light may be applied to the data-recording layer through the light-transmitting layer, thereby to record and reproduce data on and from the data-recording layer.
2. Description of the Related Art
Optical discs have been commonly used. Each optical disc comprises a data-recording layer, a reflection layer and the like that are laid on upon another. Light may be applied to the data-recording layer to record data. Attempts have been made to enhance the recording density of optical discs, in order to record as much data as possible on each optical disc.
The density at which data is recorded on an optical disc can be increased by taking two measures. First, a laser beam having a short wavelength is applied to the optical disc incorporated in a recording/reproducing apparatus. Second, an objective lens having a large numerical aperture (hereinafter referred to as “NA”) is incorporated into the recording/reproducing apparatus. More specifically, a semiconductor laser made of nitride semiconductor InGaN and emitting a laser beam having a wavelength of about 400 nm may be utilized, and an objective lens having NA of 0.8 or more may be used, thereby to increase the storage capacity of the optical disc.
A write once optical disc, which is one type of a conventional writable optical disc, is known. As shown in FIG. 1, the write once optical disc 100 comprises a transparent substrate 101 having a thickness of, for example, 1.2 mm. A guide groove 102 is cut in one major surface of the substrate 101, providing a signal-recording region. A data-recording layer 103 is provided on the signal-recording region, a reflection film 104 is formed on the guide groove 102, and a protection film 105 is laid on the reflection film 104. Thus, the optical disc 100 is a multi-layered one.
To record data signals on the write once optical disc 100, a laser beam 111 focused by the objective lens 110 for optical pickup incorporated in a recording/reproducing apparatus (not shown) is applied to the guide groove 102 through the substrate 101. Any part of the data-recording layer 103, irradiated with the laser beam 111, is heated and degenerated. A record pit (not shown) that represents a data signal is thereby made in the data-recording layer 103.
To read data signals from the data-recording layer 103, a laser beam 111 that has lower intensity than the beam applied to record data signals is applied to the signal-recording region. The presence or absence of a record pit is detected from the intensity of the beam reflected from the signal-recording region. A reproduced signal that represents the presence or absence of a record pit is thereby generated.
As indicated above, the write once optical disc 100 has a substrate 101 that is 1.2 mm thick. The laser beam 111 is applied through the substrate 101 to the data-recording layer 103. Generally, an objective lens having a large NA is used to increase the recording density of the optical disc. If so, the thicker the light-transmitting layer (i.e., substrate 101 in the case of the write once optical disc 100) through which a laser beam passes, the more greatly the skew angle of the light-transmitting layer will influence the shape of the beam spot. It follows that the thickness of the light-transmitting layer should be reduced to record data on the optical disc at highly recording density.
To accomplish high-density recording by using an objective lens of a large NA and by applying a laser beam of a short wavelength, such a rewritable optical disc 120 as is shown in FIG. 2 has been proposed. The rewritable optical disc 120 comprises a substrate 121. A guide groove 122 is cut in one major surface of the substrate 121, forming a signal-recording region. A data-recording, layer 123 is provided on the signal-recording region, and a light-transmitting layer 124 is laid on the data-recording layer 123. A blue laser beam 125 having a wavelength ranging from 380 nm to 450 nm is applied to the data-recording layer 123 through the light-transmitting layer 124.
The light-transmitting layer 124 may be made of ultraviolet-curing resin. Alternatively, the layer 124 may be a transparent film of polycarbonate or the like, adhered to the data-recording layer 123 with an adhesive agent. The light-transmitting layer 124 is preferably 10 to 177 μm thick if the objective lens used has an NA of 0.78 or more.
Data can be recorded on the rewritable optical disc 120 at a high density when a laser beam is applied to the layer 123 through the light-transmitting layer 124 that is thin. This is because the objective lens has a large NA and the laser beam applied has a short wavelength.
The data-recording layer of the rewritable optical disc 120 may be made of phase-change recording material such as Ge—Sb—Te or Ag—In—Sb—Te.
If made of phase-change material, the data-recording layer has a large absorption coefficient. Therefore, the data-recording layer cannot acquire, in principle, a high reflectance. Optical discs having a data-recording layer made of phase-change material need to exhibit a reflectance of about 15% to 25% before data is recorded on it. In view of this, the next-generation, high-density optical discs should be standardized to exhibit low reflectance.
In recent years, however, inexpensive write once optical discs having a recording layer made of organic material are more demanded. Organic materials are being developed for optical discs to which blue laser beams are applied to record and reproduce data.
As described above, the rewritable optical disc 120 needs to exhibit reflectance of about 15% to 25% (hereinafter called “initial reflectance”) before data is recorded on it. It is desired that write once optical discs to which blue laser beams are applied should have an initial reflectance of 15% to 25%, too. An optical disc may acquire an initial reflectance of 15% to 25% if the organic material of its data-recording layer has a refractive index n of 2.3 or more, as is disclosed in Japanese Patent Application No. 2000-086687.
However, few organic materials are available, which have a refractive index of 2.3 or more and which can therefore be used as material of data-recording layers. Some organic materials have indeed a refractive index of 2.3 or more. However, they are not sufficiently resistant to heat or weather, do not exhibit an absorption spectrum suitable for the wavelength of blue laser beams. Nor are they amorphous enough to inhibit noise.
In consideration of the manufacturing cost and mass-productivity of write once optical discs, it is desirable to bond a light-transmitting layer to the data-recording layer made of organic material, by applying an adhesive. If a light-transmitting layer is bonded to the data-recording layer, however, the organic material may diffuse into the adhesive or may react with the adhesive. Consequently, the data-recording layer will deteriorate. This is probably because the organic material dissolves into the solvent contained in the adhesive, such as acetic acid polymer. It is therefore required that a dielectric part be formed on the data-recording layer, thereby to prevent the adhesive from diffusing into the data-recording layer.
The data-recording layer may be made of organic material that has an refractive index of less than 2.3. In this case, the freedom of choosing the material increases. However, the optical disc can no longer acquire an initial reflectance of 15% to 25%. To raise the initial reflectance of optical discs to 15% to 25%, a dielectric part may be formed on the data-recording layer, causing multiple interference to attaining an optical enhancement effect. In order to achieve an optical enhancement effect, the dielectric part should be made of material that has so small a refractive index as possible.
As mentioned above, a dielectric part must be formed on the data-recording layer and the data-recording layer must have so small a refractive index as possible and be so high a density as possible. This holds true no matter whether the data-recording layer is made of an organic material that has a refractive index of at least 2.3 or an organic material that has a refractive index of less than 2.3.
Examples of materials that have small refractive indices are fluorides such as MgF2 and oxides such as SiO2. Films of these materials can be formed by RF-plasma sputtering. Alternatively, MgF2 films can be formed by vapor deposition, and SiO2 films can be formed by electron-beam vapor deposition. If these methods are employed to form dielectric parts of MgF2 or SiO2, however, the dielectric parts cannot be formed fast. Consequently, the optical discs cannot be mass-produced as efficiently as is desired. The methods specified above can hardly provide dielectric parts that have a desired composition. That is, the dielectric parts thus formed exhibit but insufficient transparency and inadequate surface smoothness.
Reactive sputtering that uses a silicon target and oxygen plasma may be carried out to form an SiO2 film. This method can indeed form an SiO2 film at a sufficiently high speed. However, the organic substance used is degenerated due to the oxygen plasma that is generated during the process of forming the SiO2 film. This inevitably deteriorates the data-recording layer 3.
For the reason given above, it is difficult to form a fluoride such as an MgF2 film or an oxides such as an SiO2 film, directly on the data-recording layer.